In competition cycling, weight is a critical issue. Extremely lightweight structures and structural components are used in the most serious competition bicycles. These lightweight components must be designed for a variety of very severe road and off-road loading environments. This results in a design that must operate at relatively high stress levels, close to the strength limits of the materials being used. The demand for a minimum weight bicycle has led the industry into the use of modern, high performance structural materials, such as advanced composites. These materials, although among the most efficient structural materials in existence today, impose serious demands on the design of fittings and their connections to the primary structure.
In a highly stressed part, the critical area of the design usually involves fittings and their connections. The stress distribution in this part of the structure is generally quite complex. This is especially true for bonded connections involving a high strength, fiber reinforced, composite (laminated) primary structure. Secondary stresses involving both in-plane and out-of-plane bending components can be introduced in this area of the structure, even when the applied load is a basically axial. These secondary stresses can become a significant factor in the ultimate strength and the fatigue life properties of the joint. This is due, at least in part, to the fact that: a) the applied loads at these points in the structure are very concentrated, b) the localized stress amplification factors for the various load paths leading to the primary structure are usually quite high and c) localized, secondary "hoop" and meridional tension stresses can be introduced into the joint.
This invention addresses the strength problem in bicycle dropout fittings by optimizing the grain structure of the fitting material and the fitting geometry, by minimizing the stress concentration factors in the bonded joint connection and by stabilizing the bonded surface by an appropriately shaped fitting. The result is a design that is more producible, lighter and stronger than existing prior art designs.